Coiled tungsten filaments are now used in incandescent and fluorescent lamps as well as other type electric lamps and a serious problem associated therewith occurs when these lamp filaments are manufactured. Specifically, these tungsten lamp filaments are customarily coiled and heat-treated on a wire mandrel at temperatures sufficiently elevated to produce contamination from the mandrel material. Such contamination can be avoided by controlling the temperature of heat treatment below the range where contaminant transport to the filament occurs. Thus, temperatures are generally maintained below 1100.degree. when a steel mandrel is used and 1600.degree. when a molybdenum mandrel is used. Such temperatures are close to the safe limit because of the need to maximize stress relaxation in the filament material. The coating of steel mandrels with certain metals has also been proposed but with said coatings consisting of non-refractory metals having melting temperatures considerably below the melting temperature of the tungsten filament. The coating of steel wire mandrels in this manner is said to permit heat treatment of tungsten filaments coiled thereon to temperatures around 1300.degree. C. which is sufficiently elevated to at least permit some annealing of the tungsten filaments and the coated wire mandrels are said to be dissolved thereafter in a customary manner.